Concerning a method for producing cold-rolled steel sheets, there is provided a method in which thin slabs, the thickness of which is 2 to 10 mm, are made by a twin drum type continuous casting apparatus and used as hot-rolled sheets as they are. Also, there is provided a method in which the above thin slabs are subjected to acid cleaning to remove scale from the surfaces of the slabs, and then the thin slabs are cold-rolled to a predetermined thickness and annealed.
The most important point of the above technique is the physical property of the thin slab made by the twin drum type continuous casting apparatus. According to the above conventional production process, the metallic structure of the thin slabs is coarse before cold rolling (as cast). Therefore, the thus obtained products are applied only to low grade uses. In order to improve the quality of the products, it is necessary to increase a ratio of reduction of cold rolling.
In order to obtain a fine metallic structure, the following methods are disclosed. Japanese Unexamined Patent Publication No. 61-99630 describes a method for producing cold rolled steel sheets in which: a carbon content in molten steel is adjusted to an amount of not lower than 0.015%; a thin steel strip used for cold rolling is directly cast from the above molten steel; after coagulation, the steel strip is cooled to a temperature not higher than 800.degree. C.; the steel strip is reheated to a temperature not lower than 900.degree. C.; the steel strip is cooled again to a temperature not higher than 800.degree. C.; the cooled steel strip is coiled; and the steel strip is subjected to acid cleaning, cold rolling and annealing. Japanese Unexamined Patent Publication No. 60-30545 describes a method for producing cold-rolled steel sheets in which: a continuous casting apparatus is used which has two water-cooled rollers arranged horizontally in parallel with each other while a clearance corresponding to the thickness of a metallic sheet is formed between them, rotated in the different direction to each other; a metallic sheet cast by the above apparatus is naturally cooled to a temperature not higher than the transformation point A.sub.1 ; the metallic sheet is heated to and kept at a temperature not lower than the transformation point A.sub.3 on the line; and the metallic sheet is cooled by gas or a mixture of gas and water.
However, length of the apparatus to which the above methods are applied is long because a long period of time is required for the heat treatment in the above apparatus. For example, in the example described in Japanese Patent Application No. 59-226515, operation is conducted as follows. A slab that has been cast by the apparatus is coagulated to the thickness of 3.2 mm; the coagulated slab is cooled by water to 700 to 950.degree. C.; the slab is reheated by direct heating burners for 100 seconds; the slab is kept at 950.degree. C. for 5 seconds; and the slab is coiled while it is cooled to the minimum temperature of 550.degree. C. In this case, the operating conditions are set as follows. The casting speed, by the twin drum method, is approximately 30 m/min; the water-cooling speed to cool the slab to the temperature of 700.degree. C. is 50.degree. C./sec; the reheating time at 950.degree. C. is 100 seconds; and the water-cooling speed to cool the slab to 550.degree. C. is 50.degree. C./sec. Then, the length of the apparatus of cooling--heating--cooling can be expressed by the following equation. ##EQU1##
The meaning of Equation (4) is described as follows.
(1) The first term on the left side of Equation 4 expresses the length of the apparatus required for cooling, that is, the length of the apparatus required for cooling is calculated when the period of time (min) required for cooling the slab from 1100.degree. C. to 700.degree. C. is multiplied by the casting speed (30 m/min).
(2) The second term on the left side of Equation 4 expresses the length of the apparatus required for reheating, that is, the length of the apparatus required for reheating is calculated when the period of time (min) required for reheating the slab from 700.degree. C. to 950.degree. C. is multiplied by the casting speed (30 m/min).
(3) The third term on the left side of Equation 4 expresses the length of the apparatus required for cooling, that is, the length of the apparatus required for cooling is calculated when the period of time (min) required for cooling the slab from 950.degree. C. to 550.degree. C. is multiplied by the casting speed (30 m/min).
In the example described in Japanese Patent Application No. 60-30545, when the thickness of the slab is 3 t, the casting speed is 28 m/min, and the heating time to heat the slab from a range of 650 to 700.degree. C., to a range of 900 to 950.degree. C. is 1 to 2 min. The cooling speed is 5.degree. C./sec when the slab is coiled at the coiling temperature of 700.degree. C. Then, the length of the apparatus of cooling--heating--cooling can be expressed by the following equation. ##EQU2##
The meaning of Equation (5) is described as follows.
(1) The first term on the left side of Equation 5 expresses the length of the apparatus required for cooling, that is, the length of the apparatus required for cooling is calculated when the period of time (min) required for cooling the slab from 1100.degree. C. to 700.degree. C. is multiplied by the casting speed (28 m/min).
(2) The second term on the left side of Equation 5 expresses the length of the apparatus required for reheating, that is, the length of the apparatus required for reheating is calculated when the period of time (2 minutes) required for reheating the slab is multiplied by the casting speed (28 m/min).
(3) The third term on the left side of Equation 5 expresses the length of the apparatus required for cooling, that is, the length of the apparatus required for cooling is calculated when the period of time (min) required for cooling the slab from 950.degree. C. to 700.degree. C. is multiplied by the casting speed (28 m/min).
On the surfaces of the slabs produced by the above apparatus, there are irregularities, that is, the surface conditions of the slabs produced by the above apparatus are different from those of the hot-rolled sheets produced by a conventional hot rolling mill. Therefore, the use of the slabs produced by the above apparatus is restricted. It is an object of the present invention to shorten the length of the apparatus for producing thin slabs, so that energy can be saved in the process of production. It is another object of the present invention to improve the surface roughness of the slab and make the crystal grain size of the slab to be fine.